1. Field of the Invention
This invention relates to natural zeolite adsorbents, and more particularly to adsorbents and purification by adsorbents employing natural zeolite containing iron impurity below predetermined levels.
2. Description of the Prior Art
It is well known that zeolite adsorbing agents such as molecular sieves or the like exhibit excellent adsorption performance. When using zeolite as an adsorbent, it is desired that the adsorbent be low in manufacturing cost, excellent in adsorption performance and capable of repeated re-activation for long use. For the reduction in manufacturing cost, it is desired that natural resources be used as effectively as possible; for the capability of repeated re-activation, it is desired that the material used be excellent in resistance to acid and heat; and, for high adsorption performance, the material should have high intrinsic adsorption potentials and it is desired that some definite criteria be set up for ascertaining the suitability of the raw zeolite ores for use as the adsorbent in terms of its impurity content.
Up to now, however, substantially no satisfactory zeolite adsorbents or methods for selecting zeolite minerals therefor have been proposed which meet such requirements as mentioned above. In the past, use has been made of relatively expensive synthetic zeolites commonly referred to as molecular sieves.
Japan and some other countries are abundant in resources of natural zeolite of good quality (especially mordenite and clinoptilolite), and it is highly desired to make effective use of natural mordenite and natural clinoptilolite. In connection with the natural zeolite, however, problems arising from its composition, impurity content and so forth with respect to the adsorption performance have not been made clear and solved yet, so that the natural zeolite has hardly been used as a high quality desiccating agent for it retains unsolved problems in the quality and reliability as a desiccating agent.
Of the natural zeolite resources, analcite, mordenite and clinoptilolite are available relatively cheaply and abundantly. In particular, mordenite and clinoptilolite have already been exploited commercially and employed for various purposes. Mordenite and clinoptilolite are very similar in chemical composition to each other, and their Si/Al ratios, i.e. their silicon to aluminum atomic ratios, are both about 4 or more, which is very high for use as zeolite; they are stable chemically and have excellent acid and heat resistances. Further, natural mordenite has a high degree of mechanical strength. The Si/Al ratio may sometimes be represented in the term of molar ratio and, in such a case, the SiO.sub.2 /Al.sub.2 O.sub.3 molecular ratio becomes twice the Si/Al atomic ratio.
The potential capacities of natural zeolite as an adsorbent have been recognized, but natural zeolite is usually low in purity and poorer in adsorption performance than the synthetic zeolite and a proper, convenient ore-dressing method has not been established, so that the use of natural zeolite as adsorbing agents is much restricted. For example, since a pelletized catalyst of the synthetic mordenite is low in mechanical strength even when some binder material such as clay is added, natural mordenite may in some cases be added to the synthetic mordenite so as to provide for increased mechanical strength.
Molecular sieves A and X which are well-known synthetic zeolites are low in acid resistance and in heat resistance. Their crystal structures are changed by high-temperature heating and destroyed by acid. Accordingly, they cannot be re-activated at high temperatures and cannot be used in an acidic solution or in an acidic atmosphere. Low temperature re-activation will lead to decrease in adsorption due to imcomplete re-activation. In contrast thereto, natural mordenite and natural clinoptilolite have a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio, and hence are stable in crystal structure and high in acid resistance and in heat resistance.
A proper classification of natural zeolite to guarantee a high degree of adsorption performance coupled with the abovesaid advantages, will broaden its application.
Description will be given of some technical fields to which natural zeolite can be applied. In the fields, for example, of chemical and of semiconductor industries, there are often needed fluids free of undesirable impurities such as water vapor or the like. This happens, for example, in the case of filling a high-temperature reaction furnace with an inert gas or hydrogen gas for preventing an unwanted chemical reaction between the reactant and the furnace material being heated. In such a case, the lowest guaranteed dew point is more important than the maximum amount of water which the adsorbent can adsorb. When the adsorbing agents are to be repeatedly re-activated for long use, the re-activation efficiency, i.e. to what extent the adsorption performance can be revived, is important if the initial maximum adsorption performances are the same. For complete release of adsorbed water, a high re-activation temperature is desired, but molecular sieves 4A and the like have the defect of low heat resistance.
It is known that a very small amount of water content greatly influences the quality of products in the crystal growth of a material free from water of crystallization, in the substrate treatment for vacuum evaporation, in the fabrication of glass fibers and go forth.
Where it is desired to remove as much water as possible, if the manufacturing process involves treatment in a fluid, much care is necessary in pretreatment of the fluid. If any water remains in the fluid, then it will adhere very easily to the surface of an article being treated and will not be readily removed therefrom.
Well-known desiccating agents are active carbon, silica gel, molecular sieves (synthetic zeolite). As higher desiccating agents for industrial use, molecular sieves NaA and the like are often employed partly because of their high adsorption performance and partly because of availability of chemically stable ones of constant quality, but their adsorption performance is not very satisfactory.
When used as desiccating agents, the molecular sieves are usually re-activated by heating for repeated use, but the adsorption performance of ordinary molecular sieves decreases with an increase in the number of re-activations; accordingly, they cannot be re-activated too many times if one desires very high adsorption performances. The cause of this shortcoming is considered to arise chiefly from insufficient removal of adsorbed substances due to low re-activating temperatures. The ordinary molecular sieves are poor in their heat resistance and cannot be re-activated by high-temperature heating. Accordingly, when using the molecular sieves are desiccating agents, the cost is relatively higher than in the case of utilizing other materials.
The adsorption performance of zeolite varies from material to material to be adsorbed; for example, it adsorbs water well but does not equally oxygen. Accordingly, the zeolite adsorbent is effective for gas dehydration but may not work satisfactorily for the removal of oxygen. As a consequence, in the case of purifying a hydrogen gas, use is often made of an adsorbing agent utilizing an expensive palladium film. The palladium film is not only expensive but also poor in mechanical strength and would easily be broken by erroneous temperature control or like operation.
Also in many other technical fields, highly efficient, long-lived, inexpensive adsorbing agents are desired.
In contrast to the removal of small impurities, it is also desired in other fields to reduce the impurity content below a certain level at low cost and with ease; for example, in the cases of purification of oxygen gas for welding use, a foreline trap for water or oil mist is used in a vacuum equipment and in treatment of waste liquid and waste gas in various fields. It is highly desired that these operations neither require excessive human handling nor greater cost. The use of synthetic zeolite adsorbents may often pose problems in their service life, cost and so forth.
As will be appreciated from the above, in the technical fields utilizing the zeolite adsorbents, there is a strong demand for zeolite adsorbents which have higher adsorption performance and longer service life and are less expensive. If properly classsified, natural zeolite would fill such demand.